This study proposes a single-stage solar photovoltaic energy conversion system (PV-ECS) integrated to a three phase four-wire (3P4W) distribution grid with dual-function capabilities, i.e. active power transfer and power quality (PQ) enhancement at the point of interaction (PoI). The PV-ECS system comprises of a solar photovoltaic array and a voltage source inverter (VSI), supplying active power (during daytime) to the distribution grid and connected single-phase and three-phase loads. Apart from transfer of power, the system also improves the PQ at the PoI by compensating reactive power and neutral current, attenuating harmonics, correcting power factor and balancing grid currents. During night, the VSI acts as a shunt active power filter mitigating PQ issues, thereby increasing the device utilisation factor. A three-phase magnitude-phase locked loop (3M-PLL) method is utilised to extract and estimate fundamental term of load currents and an incremental conductance algorithm is applied for maximum power point tracking. To demonstrate its effectiveness, the system is modelled and its performance is simulated on MATLAB and experiments are performed on a developed prototype in the laboratory.
Fig. 1 System configuration and control scheme
(a) Structure diagram of 3P4W grid-connected PV-ECS
EXPECTED SIMULATION RESULTS
Fig. 2 Dynamic behaviour of system at
(a), (b) Unbalanced load, (c) Step increase in irradiance from 700 to 1000 W/m2
A dual-function single-stage PV-ECS integrated to the 3P4 distribution grid has been proposed here. Two modes of operation of PV-ECS are to supply and transfer active power to the grid and tied loads as well as to improve quality of power at PoI. An In Cbased approach is utilised here for tracking MPP of solar PV array and a 3M-PLL-based control scheme is utilised for extracting fundamental components of load current. Simulated and test results have demonstrated the performance of the system under various conditions such as non-linear loading, unbalanced loading and varying irradiance levels. Test results have shown that the system has improved the power quality at the PoI by compensating neutral current and reactive power, correcting power factor and balancing loads on the grid side. The harmonics are reduced to below 5% on grid side, which is within the limits of an IEEE-519 standard. Moreover, test results have indicated that the system has operated suitably during night-time (sunlight unavailability) thereby increasing the utilisation factor of the VSI by two-fold. The single stage structure has decreased the losses in the system and increased the total efficacy of the system.
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